But in that case we’d have to define the JTS moment not as the moment the show started to decline in quality but a moment later on when the show has been declining for a while and a show is put on which is particularly bad. Then what do we call the moment when the show’s decline begins?

Take The Simpsons. Let us suppose that people are arguing that the show isn’t as good as it used to be. (Which may well be true, I haven’t watched in a while.) The group attempts to put a date on the moment that the decline in quality began. Anything before that moment would be taken as the show’s peak, anything post that moment part of the decline. They fix on a particular show and declare that is when The Simpsons jumped the shark, when the rot began. That’s how I’ve always understood the phrase.

I think the common understanding of the JTS moment is the moment when no reasonable person could deny that the show has gone past its peak. What its meaning may have been in the mind of the coiner of the phrase may be relevant to a historical discussion of the phrase, but it is pretty much irrelevant to its common use.